In the last several years there has been an explosion in the public's awareness of the importance of physical activity. Commensurate with this explosion has been a flurry of technological advances in the field of athletic footwear. Many of these advances relate to either the fit or the function of footwear. One area of footwear development which has not been adequately addressed is that of reducing the weight of an athletic shoe.
A conventional athletic shoe typically includes a number of components which have been traditionally considered to be "essential" elements of athletic footwear. Included in these "essential" components is the outsole. The outsole of an athletic shoe is the ground engaging portion of the shoe. As such, the outsole is typically made of an abrasive resistant material such as rubber. Because it is critical that the outsole exhibit certain wear resistent characteristics, there are a finite number of materials from which to make an outsole.
Another element of a conventional athletic shoe is the midsole. The midsole is that portion of the shoe which is primarily responsible for cushioning. While recent years have brought many variations to midsole design such as the use of inserts, the principal materials used to supply cushioning include polyurethane (PU) and ethylvinyl acetate (EVA) foams. More recently, foams made of HYTREL.TM. have been used as the cushioning material for midsoles. HYTREL.TM., manufactured by E.I. DuPont de Nemours & Company, Inc., is a semi-crystalline, fully polymerized, high molecular weight elastomer composed of alternate amorphous and crystalline chains. In many athletic shoes, a contoured foam sockliner is disposed above the midsole to provide additional cushioning and support.
Finally, the upper, that portion of the athletic shoe which surrounds and protects the foot, is an element which is typically found in footwear.
With little exception, it is a goal of an athletic shoe maker to produce a shoe which is as light as possible. However, the shoe maker is faced with competing interests which oftentimes require sacrificing weight for another required characteristic of the shoe. For instance, if an athletic shoe is to provide sufficient support to the ankle, the athletic shoe must incorporate materials having sufficient density to accomplish the desired effect. Some inroads have been made in this regard by the use of advanced cushioning technologies. Similarly, it may be necessary to use a specific density of PU or EVA in the midsole to provide adequate support and cushioning to the wearer. One technique used to reduce the weight of a shoe is to reduce the density of the material used to form the midsole. To support the low density midsole material, a mechanical insert may be embedded in the lower density material to provide the structural support necessary to provide adequate stability.
While mechanical frames and inserts in many cases serve to provide a structure to store and return energy to the midsole, the effectiveness of these structures is primarily in the mechanical performance of the midsole and not in weight reduction. In short, while these structures perform an important function in the performance of athletic footwear by serving to maintain the structure of the midsole and to help the midsole to recover from the application of a force, they do not serve to substantially reduce the weight of an athletic shoe.
One technique which has been used to reduce weight in an athletic shoe is to remove those portions of the outsole which are not needed. Many athletic shoes on the market have either openings in the outsole or have had outsole material removed in certain regions. The removal of this material serves to reduce the weight of shoes but not nearly to the extent possible.
More recently, some companies have taken the additional step of removing material not only from the outsole but also from the midsole. While removing material from the midsole is effective to reduce the weight of a shoe, this course can not be taken haphazardly. The prior art has not recognized those regions of the foot in which material can be fully removed.
In order for a shoe to take into account the structural anatomy and requirements of the foot, the physiological aspects of the foot must be understood. The human foot is a complex structure which depends on twenty-six bones to work in conjunction with each other to support the weight of their owner and to transport the body under a number of different conditions. The tapestry of bones which make up the foot not only supports the weight of the body but withstands forces of perhaps 250% of normal body weight which can occur, for example, during jogging.
The foot can be broken down into three basic functional segments. The posterior segment is that segment underlying and supporting the tibia. This segment includes the talus which directly underlies the tibia and the calcaneus, that portion of the foot which will typically make first contact with the ground during a normal gait cycle. The second segment is simply known as the middle segment. The middle segment of the foot contains five tarsal bones which are arranged in a complex geometrical formation. The third segment is the anterior segment which includes five metatarsal bones and fourteen phalangeal bones (three for each toe with the exception of the great toe which has two).
An analysis of the movement of the foot allows footwear to be produced with a structure that is commensurate with the requirements of the foot. Ignoring the requirements of the foot results in footwear that provides adequate cushioning only by utilizing cushioning material for virtually every structural component of a shoe. The result is a well cushioned (but heavy) shoe.
Human movement or locomotion is the translation of the body from one location to another. This movement occurs in a remarkably efficient manner. The particular way or fashion of moving on foot is called "gait." All the determinants of gait tend to minimize the amount of energy it takes to move from one location to another. Thus, the body works to minimize all forces which tend to impede the movement. The foot works with the remainder of the body to move in the most efficient way possible. During walking, there are two general phases, the swing phase (when the foot is not in contact with the ground) and the stance phase (when the foot is in contact with the ground). The stance phase is the important phase to analyze to determine what physiological criteria must be utilized in order to make the optimal shoe. Interestingly, it is the swing phase which drives the development of the present invention in that it is during this phase that it is critical to have footwear with a minimum possible weight. In order to determine how to reduce the weight of the shoe, the stance phase must be analyzed.
The stance phase of the gait cycle begins with heel strike. During heel strike the foot is typically in a supinated orientation. The foot pronates and the metatarsal heads make contact with the ground. At this mid-stance orientation the weight of the body bears primarily at the heel and at the metatarsal heads. The heel then lifts in the heel-off phase of the gait cycle. Finally the stance phase ends with toe-off after which the foot again supines. A similar cycle occurs during jogging.
In short, the contact of the heel with the ground is typically referred to as heel strike. The weight of the person proceeds along the lateral border of the foot toward the metatarsal heads with the major propulsion thrust by the distal phalanx of the great toe.
Whether a person is running, walking, jogging or simply standing, there are certain cushioning requirements. Typically, the prior art has incorporated cushioning under the entire foot, including the arch. However, with the weight of the person shifting along the lateral side of the foot, cushioning to the extent found in athletic shoes is simply not needed in the arch.
The arch of the foot is formed in part by the orientation of the bones. The bones are oriented in substantially a bow shape with the calcaneus forming one end of the bow, the metatarsal heads forming the other end of the bow. Forces are applied to the bow at a point (at the talus) between the calcaneus and the metatarsal heads. Thus, when downward force is applied to the talus, there are equal and opposite forces applied at the two ends of the bow. A muscle known as the planter fascia extends from the medial tubercle of the calcaneus to the toes. The planter fascia runs anteriorly from the calcaneus and splits into five bands, one attached to each toe. During running tremendous stress can be placed on the fascia planter. Simple mechanics dictate that when a downward force through the talus force is placed at the calcaneus and at the metatarsal heads, as is the case during most activities, the planter fascia tends to want to stretch. One of the objects of the present invention is to support the arch of the foot and to at the same time make a lighter shoe than those previously known in the art.
Accordingly, one of the features of the present invention is that it provides an athletic shoe of reduced weight. Because of the unique aspects of the present invention, the reduction in weight may be made without adversely effecting the performance of the athletic shoe. Additionally, one feature of the present invention is that it enables a number of different technologies to be used in conjunction with the invention. Additional objects, advantages, and novel features of the invention will be set forth in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned from practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations pointed out below.